1. Field of the Invention
This invention relates to a surface acoustic wave element and, in particular, to an improvement of a monolithic surface acoustic wave element made of a piezoelectric layer and a semiconductor.
2. Description of the Prior Art
A surface acoustic wave (SAW) element has an important role as a signal processing device because of its high frequency signal storage function and batch processing function. Moreover, when a monolithic integrated device is made by incorporating the surface acoustic wave element and a semiconductor device on a single substrate has additional various functions, such as amplification, multiplication and so forth, which are realized by the semiconductor element, it is becoming multifunctional.
There are two current proposals for arrangement of monolithic multifunctional integrated SAW devices. One of them is a multi-layered structure forming a piezoelectric thin film of ZnO, AlN, etc. on a Si semiconductor substrate, and the other is a structure incorporating an SAW device and a semiconductor device on a GaAs substrate which is a piezoelectric semiconductor.
A III-V group compound semiconductor such as GaAs has the following electric, acoustic and optical properties as compared to Si:
a) Electron mobility is as high as several times the value of Si. PA1 b) Electric conductivity can be controlled over a wide range from a semi-insulator to a conductor. PA1 c) It has a piezoelectricity. PA1 d) An element which can emit or receive wide range of light from visible rays to infrared rays.
In addition, because of recent remarkable development of technology for making a III-V group compound semiconductor thin film, it has been possible to form a GaAs/AlGaAs or other element having a hetero structure, and a device utilizing quantum effects is also appreciated to have excellent functions.
As discussed above, GaAs has many functions that Si does not; however, the multifunctional property of an element has come to a limit in the existing composite devices of Si and a piezoelectric thin film such as ZnO, AlN, etc.
Problems with an SAW device using GaAs is a high cost of its substrate caused by bad productivity because it is still difficult to produce wafer with a sufficiently large diameter.